More than half a decade after their identification, the biological role of TCRgammadelta cells remains uncertain. While TCRgammadelta cells are clearly dominant in certain pathologic conditions, their lack of significant involvement in most classical T cell responses suggests that they perform a unique and special role in host immune defense. In fact, TCRgammadelta cells have several unique features that distinguish them from TCRalphabeta cells including: (a) preferential expression in epithelial tissue; (b) a broad repertoire skewed towards non-classical major histocompatibility complex (MHC) class I antigens, heat shock proteins, and non-processes antigens; and (c) substantial expansion during a variety of inflammatory diseases thus constituting a major arm of the immune system. The major objective of the previous grant was to dissect and functionally characterize this unique T cell subset. During this time, we identified TCRgammadelta cells showing that TCRgammadelta cells can be MHC-specific (classical class I-, class II-, and TL-specific) and nominal antigen- specific. The antigen-specific TCRgammadelta cells were used: 1) To demonstrate that the TCRgammadelta repertoire is diverse; 2) To create TCRgammadelta transgenic mice that establish positive and negative selection in TCRgammadelta development; 3) To produce multiple antisera and mABs that have been widely used to characterize TCRgammadelta subsets and examine the functional potential (i.e. lymphokine production and cytolytic activity) of this novel T cell population in normal and transgenic mice. During the next grant period, we will use the mABs, T cell clones and transgenic mice to define TCRgammadelta cell activation and intracellular signal transduction pathways, the physiologic processes that affect positive and negative selection, and the genetic elements that control antigen recognition by this eclectic population. Specifically, we will: 1) Study the precise antigen specificity and physiologic role of herpesvirus-specific and TL-specific TCRgammadelta cells. We will examine the role of antigen processing and the requirement for peptide- presentation in the specificity of these T cells; 2) Examine the biochemical and molecular events associated with TCRgammadelta maturation and selection. We will examine directly the TCR complex and early signal transduction pathways in developing TCRgammadelta cells and during antigen-driven activation; and 3) Determine the genetic elements that control TCRgammadelta repertoire development. We will identify the genes and antigens that control strain-specific repertoire differences; These studies will help to define the role of TCRgammadelta cells in immune responses.